1. Field of the Invention
The present invention concerns cohesive finishes for materials containing resin, particularly including finishes made from resin-impregnated fabric the resin of which fabric is cohesively bonded to the resin of fiber-reinforced composite materials.
DESCRIPTION OF THE PRIOR ART
The present invention is concerned with the application of a finish, or surface layer, to an article made from a certain class of materials, and with the finish so obtained. Generally within the prior art finishes are applied to articles for many purposes. These purposes include coloring, patterning, and/or altering the surface texture of the article. These purposes also include sealing the surface of the article, improving the article's immunity to weather or other environmental agents, imparting strength and durability to the article, and generally altering selected physical properties of the article. Finishes may particularly serve to provide a surface layer which is distinguished from the underlying material by being harder, impervious to certain agents, colored, patterned, or otherwise differentiated from other regions of the article to which the finish is applied. Painting is well known as a common means of applying a finish to a surface in order to improve the properties of the surface.
Paint and other common finishes are characterized by being adhered, or in adhesion, to the underlying material. Adhesion is the molecular attraction exerted between the surfaces of bodies in contact. A body having an adhered finish, for example paint, is characterized in that at some regions sufficiently distant from the surface there exists only the material of the body whereas at the uppermost regions of the surface there exists only the paint. Adhesion is to be contrasted with cohesion, meaning molecular attraction by which the particles of a body are united throughout the mass. Generally a cohesive body may exhibit differentiation between surface and underlying volumes but this differentiation is not complete, and both the surface and subsurface volumes are intimately molecularly interconnected (often gradually and progressively) and share certain properties.
There is a class of articles for which prior art adhered finishes have not proven to be completely satisfactory. This class of articles are made from the so-called "advanced composite materials", which are resin materials reinforced with fibers such as those of glass, carbon, metal or aramid, or with ceramic filaments. Problems with the finishing of articles made from advanced composite materials are rooted in the exceptional properties of these materials, which properties must be matched if not enhanced by any adhered finish. These properties include strengths exceeding those of high strength steels, stiffness matching that of steel at less than two-thirds the weight of aluminum, fatigue resistance, creep resistance, thermal dimensional stability, wear resistance, damage tolerance, corrosion resistance, and vibration resistance.
Articles made of the advanced composite materials are widely used in exceedingly demanding applications. These applications include aircraft and spacecraft components such as fuselages, ailerons, spoilers, flaps, structural sections, doors, rotor blades, rotor shafts, and fuel tanks. These applications further include vehicular components such as body panels, structural members, shafts, connecting rods, springs, and fuel tanks. These applications further include boat and ship components such as hulls, masts, booms, and winches. Finally, the composite materials find wide application in sporting goods such as fishing rods, tennis rackets, archery equipments, skis and ski poles, protective clothing and helmets, vaulting poles, and golf clubs. In all these applications the stringent requirements made on an article formed of advanced composite material are likewise made upon the finish of the article. The finishes commonly adhered to such surfaces have proven to exhibit problems in meeting such stringent requirements.
For example, perhaps the most common finish for articles made from advanced composite materials is gel coat, or a paint-like substance which is made from a resin, typically epoxy resin, that is compatible to adhere to the article. Structural panels of advanced composite material that are used in cars, boats, and aircraft sometimes have a gel coat of epoxy resin applied at thicknesses ranging up to one quarter inch. One limitation of this prior art gel coat finish is that it is normally not susceptible to being either textured nor patterned, but is generally only smooth and of a uniform color. Another limitation of the finish is that the finish's color is diffused throughout the gel coat and, although this color may be durable, it appears flat and without depth because it is uniform throughout the resin.
Still another important limitation of the prior art gel coat finish is that it is normally very thick and heavy. This defeats some of the advantage of light weight which may have been instrumental in the original choice of a panel made from advance composite material. This added weight is especially disadvantageous in aircraft where paint, or gel coat, thicknesses typically range to one quarter inch and more (especially on leading edges and noses) in order to obtain adequate durability of the finish to abrasion from wind, rain, dirt, and like airborne elements.
Another example of problems with prior art surface finishes to articles made from the advanced composite materials is that the dimensional tolerance of the finish is difficult to control. Variations in the depth of the finish, which is especially common and nearly unavoidable with paint or gel coat, correspondingly alters the dimensions of the article. Certain composite material articles, such as shafts, which must exhibit precise dimensional tolerances are seldom found finished because of this problem.
Related to the problem of uniformity in the depth of prior art finishes to articles made from advanced composite materials, the finishes often exhibit undesirable visual irregularities. These irregularities include undulations, areas of differing roughness and smoothness, and occasional poor area coverage. This may be due to the high general viscosity of resins (including epoxy resins), and their imperfect adaptability to achieving the same uniformity as conventional paint when applied under the same conditions.
Finishes, especially epoxy resin finishes, to articles made from advanced composite materials are often suitable to be applied only under a limited range of controlled environmental conditions. Particularly, the temperature of application and cure must normally be high room temperature ambient or above (inducing discomfort to the applier), and the curing times are often prolonged to many hours or days.
Still another example of problems with prior art finishes to articles made from advanced composite materials is that the finish may detrimentally change the mechanical qualities of the article, particularly its resistance to flexion, tension, compression, distortion, and like spatial stresses. Although finish-induced changes to an article's mechanical properties are sometimes intentionally induced in certain composite material articles such as hard armor, within certain other composite material articles such as those used in sports any finish-induced variation in the intrinsic mechanical properties of the article is considered very detrimental. Fishing rods, golf club shafts, tennis rackets, and vaulting poles in particular are of such modest relative diameters to any surface finish applied that the finish may undesirably alter the precisely predetermined and highly controlled mechanical properties of the article. An undesirable alteration in the magnitude and uniformity of resistance to the mechanical motions of flexion and extension, in particular, may result from the prior art application of an adhered finish, especially if the application is irregular or wears to become so. Flexion, extension, and resistance to torque are properties which are most desired to be established with precision, and maintained constant, in sporting articles. Thus the mechanical properties of a composite material body may actually be altered by its finish, and are often undesirably and uncontrollably so altered in the prior art.
There is sometimes a problem with the durability of finishes adhered to composite material bodies. In many finished composite material bodies that are subject to vibration, shock, torque, and/or dimensional distortion--especially including sporting goods--the adhered finish deteriorates before the underlying composite material. This is understandable since the boundary between the adhered finish and the underlying body, even if molecularly very strong, incurs extreme stress.
Finally, there are certain problems, or at least conditions, with the prior art composite material articles themselves. Sometimes the finish of the article could conceivably help the problem, although the prior art has not even perceived the finishes of such articles to be relevant to these problem conditions. This is possibly because the finishes themselves have been a problem area, and have thus scarcely been expected to contribute to the solution of other problems. One of these problem conditions occurring with some composite material articles is the well known electrically insulating qualities of the advanced composite materials, particularly the fiber-reinforced resin materials. Although the electrically insulating qualities of composite material articles are exploited to good advantage in some applications (such as components for X-ray machines), aircraft panels formed from composite material are not desired to be electrically insulating. Rather, these panels must conduct static electricity and lightning strikes upon the aircraft. Within the prior art they are presently aided to do so by the incorporation of copper wires, or grids, that are imbedded within the composite material. However, this is a poor solution because (i) electrical potential is not rendered everywhere equal on the surface of the aircraft, (ii) the insulator material may ablate if subject to a direct lightning strikes, and (iii) it is difficult and of uncertain reliability to ensure electrical connection to and through the copper wires. Because the provision of an adhered surface finish to aircraft panels has in the past presented its own problems of weight and durability, there has apparently been no consideration in the prior art regarding any contribution to be made by a panel's surface finish to the panel's electrical non-conductivity problem.
Because of difficulties in applying an adhered finish to bodies of composite material so that the finish is of equally superb physical characteristics to the bodies themselves, unfinished composite material bodies are very prevalent at the present time. This prevalence is not warranted. First, even though articles made from advanced composite materials generally exhibit excellent immunity to chemicals, including corrosives, the fibers or filaments of the composite material provide natural passageways into the bodies of the articles. These passageways can, over time, pick up contamination which degrades the article or its surface. Therefore, finishes are protective. Second, a certain public fascination with the visual appearance of articles made from the advanced composite materials, especially with the black surface of graphite fiber reinforced material in use for sporting goods, is already waning. Certain composite material bodies, such a body panels for automobiles, have no significant application in their unfinished and uncolored form, and must be finished for esthetics reasons.
Accordingly, it would be desirable if some manner of providing a finish to the surface of articles made from the advanced composite materials could be derived wherein the finish would possess equally superlative characteristics to the composite material but would still be capable of readily realizing the colors, patterns, textures; lustre, and protection which are generally associated with those lessor finishes, including common paint, which are within the prior art.